Vesicle trafficking involves transport between subcellular compartments of eukaryotic cells. During this process, vesicles bud from a donor membrane and fuse with a recipient to deliver internalized materials. Vesicular transport is regulated by a class of proteins, named Rab proteins.
Rab proteins are low molecular weight (LMW) guanidine triphosphatases (GTPases) and belong to the Ras superfamily. These proteins assist the binding of transport vesicles to their accepter organelles and initiate the vesicle fusion process using the energy from the hydrolysis of GTP. Rab proteins have a highly variable amino terminus containing membrane-specific signal information and a prenylated carboxy terminus which determines the target membrane to which the Rab proteins anchor.
More than 30 Rab proteins have been identified in a variety of species, and each has a characteristic intracellular location and distinct transport function. In particular, Rab1 and Rab2 are important in ER-to-Golgi transport; Rab3 transports secretory vesicles to the extracellular membrane; Rab5 is localized to endosomes and regulates the fusion of early endosomes into late endosomes; Rab6 is specific to the Golgi apparatus and regulates intra-Golgi transport events; Rab7 and Rab9 stimulate the fusion of late endosomes and Golgi vesicles with lysosomes, respectively; and Rab10mediates vesicle fusion from the medial Golgi to the trans Golgi.
The function of Rab proteins in vescular transport requires the cooperation of many other proteins. Specifically, the membrane-targeting process is assisted by a series of escort proteins (Khosravi-Far, R. et al. (1991) Proc. Natl. Acad. Sci. 88: 6264-6268). In the medial Golgi, it has been shown that GTP-bound Rab proteins initiate the binding of VAMP-like proteins of the transport vesicle to syntaxin-like proteins on the acceptor membrane, which subsequently triggers a cascade of protein-binding and membrane-fusion events. After transport, GTPase-activating proteins (GAPs) in the target membrane are responsible for converting the GTP-bound Rab proteins to their GDP-bound state. And finally, guanine-nucleotide dissociation inhibitor (GDI) recruites the GDP-bound proteins to their membrane of origin.
Rab-specific GDIs have a dual function of inhibiting GTP release and escorting Rab proteins from the cytosol to their target membranes. GDIs have been shown to function as cytosolic acceptors for newly synthesized and carboxy terminus-prenylated Rab5 (Sanford, J. C. et al. (1995) J. Biol. Chem. 270: 26904-26909). Several GDI isoforms have been identified in mouse using yeast two-hybrid system screening (Janoueix-Lerosey, I. et al. (1995) J. Biol. Chem. 270: 14801-14808). These proteins or fragments, GDI.beta., clone B, and clone C, are capable of interacting with either or both of Rab5 and Rab6 in the yeast two-hybrid system. X-ray structure of a Rab-GDI complex reveals that GDI has a conserved Rab-binding region and structural homology to flavin adenine dinucleotide (FAD)-binding proteins (Wu, S. K. et al. (1996) Trends Biochem. Sci. 21: 472-476).
The discovery of a new human Rab-specific guanine-nucleotide dissociation inhibitor and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of inflammation and disorders associated with cell proliferation and apoptosis.